Belts used in this type of application (which include transverse teeth on their inside faces when the special flexibility characteristics of belts are required or in certain applications requiring small winding diameters) must be capable of supporting different stress states over long periods of time, must have a high value modulus of elasticity in traction, a low value modulus of elasticity in bending, and a high value of transverse mechanical strength. This last condition is particularly important given that in the absence of adequate transverse stiffness the belt may curve in operation with its right cross-section warping under pressure from the cheeks of the pulleys with which it co-operates, thereby ceasing to transmit power. In order to avoid this drawback, proposals have already been made to increase the transverse stiffness of such belts, for example by adding short fibers uniformly dispersed through the rubber mixture constituting the matrix of the belt, with the fibers being mostly oriented in the transverse direction of the belt, thereby obtaining an anisotropic mixture having a higher value modulus in the transverse direction than in the longitudinal or radial direction. However, in such a solution the degree of transverse stiffness which can be obtained is limited by the fact that the fraction of natural or synthetic short fibers may not exceed about 30% by volume since beyond that value cohesion is rapidly lost between the rubber matrix and the fibers.
Other proposals have also been made for increasing the transverse stiffness of such belts, and in particular, European patent application EP-A-0 109 990 proposes inserting reinforcement constituted by filamentary elements extending substantially across the entire width of the belt on either side of its longitudinal strength member (or cord), i.e. both in the portion of the belt which operates in compression (the inside) and the portion which operates in traction (the outside). In this way, the belt obtains the transverse stiffness required for good compression performance, however this is to the detriment of its longitudinal flexibility which is essential for enabling it to withstand the bending to which it is subjected in operation without being damaged, i.e. without rupturing cohesion at the interface between the reinforcement and the elastomer matrix. In addition, increasing the amount of reinforcement increases cost and also increases difficulty of manufacture, while simultaneously causing contact between the flanks of the belt and the cheeks of the pulleys to create non-uniform zones having very different physical characteristics, particularly with respect to their coefficients of friction. Such differences in coefficients of friction interfere with transmission and set up zones of localized heating which damage the elastomer constituting the matrix, in particular by causing it to crack with the result that cohesion of the assembly is substantially reduced and consequently the characteristics and the lifetimes of such belts are also reduced.
On the basis of this state of the art, the Applicants have sought to provide a belt of the type mentioned above, i.e. in which transverse stiffness is obtained by means of reinforcement based on filaments, but which avoids the drawbacks of known prior art belts.
A particular object of the invention is to provide such a belt for transmitting high power without degrading the longitudinal flexibility of the belt.
Another object of the invention is to provide such a belt whose lifetime is at least as long, if not longer, than that of previously known belts.
Another object of the invention is to provide such a belt which can be manufactured without difficulty in substantially the same way as prior belts, thereby avoiding the need to create special high cost tools and/or apparatuses.